What's the best way to model organic shapes?
The current topology cannot benefit that. The basic workflow and recommended topology would be:


Create the base mesh for the sole.
Add a Subdivide Surface modifier, then apply.
Add another Subdivide Surface modifier. Tweak mesh shape with Proportional Editing.
Build the straps with operations such as Extrude, Bridge and Loop Cut.
Build other details, such as thickness.
Finally refine the shape.




Example file: